This invention relates to printing presses, to web-fed offset printing presses, and, more specifically, to a blanket cylinder for use in such presses. Still more specifically, the invention deals with improvements in or relating to such a blanket cylinder designed for easy, firm mounting of a blanket around the same as well as for ready dismounting of the blanket therefrom.
In web-fed offset lithography, as is well known, a printing plate on a plate cylinder prints the inked image on the surface of a blanket of rubber or like material fastened to a blanket cylinder. The printed image is then offset or transferred from the blanket to the web running against an impression cylinder or, in offset perfecting presses, against another blanket cylinder.
A variety of blanket cylinders have been devised and used which are designed for easy mounting and dismounting of blankets. Out of such prior art devices, two are considered pertinent to the instant invention. One of these, disclosed in Japanese Unexamined Patent Publication No. 9-99543, teaches to create in a blanket cylinder an undercut groove extending parallel to the cylinder axis. Rotatably mounted in the undercut groove is a spring-loaded spindle having a series of pusher lugs aligned longitudinally thereof at constant spacings. The spindle is driven between a working position, in which the pusher lugs are oriented normal to the wall surface of the undercut groove, and a retracted position in which the lugs are turned away from the groove wall.
The rubber blanket for use with this prior art blanket cylinder is bonded to a sheet-metal carrier of rectangular shape. The blanket carrier has a pair of opposite end portions, extending beyond the edges of the blanket, that are bent respectively into an acute and an obtuse angle whose sum is approximately 180 degrees.
For mounting the blanket to the blanket cylinder, the acute-angled end portion of the blanket carrier is first hooked onto one of the pair of longitudinal edges of the entrance opening of the undercut groove; then the blanket with the carrier is wound around the cylinder by turning the latter; and then the obtuse-angled end portion of the blanket carrier is inserted in the undercut groove. The pair of opposite end portions of the blanket carrier, bent complementarily, are now held against each other in the undercut groove. Then the spring-loaded spindle is revolved from the retracted to the working position, with the result that the pusher lugs thereon are turned into abutment against the superposed end portions of the blanket carrier, until, with the blanket tightened against the cylinder, the pusher lugs become approximately perpendicular to the carrier end portions, pushing them against the groove wall and so frictionally retaining them against displacement during printing. The spindle is to be turned back to the retracted position for dismounting the blanket.
An objection to this prior art blanket cylinder is the frictional engagement of the blanket carrier end portions by the pusher lugs on the spindle. During printing, of course, the blanket cylinder rotates in rolling engagement with the plate cylinder and, via the web of paper, with the impression cylinder or, in the case of an offset perfecting press, with the blanket cylinder on the other side of the web. The blanket tends to slacken and get loose on the blanket cylinder by reasons of its direct or indirect contact with the other cylinders. The prior art blanket cylinder is totally unequipped to take up the possible slack.
Japanese Patent No. 2,746,839 defeats the noted shortcoming of the first described prior art device, suggesting a blanket cylinder also having a groove cut in its surface so as to extend parallel to the cylinder axis. Rotatably received in this groove, a blanket-tightening spindle is toothed for mating engagement with racks each extending at right angles with the spindle axis and all arranged at spacings axially of the spindle. The racks are sprung, biasing the spindle to turn in a predetermined blanket-tightening direction.
The blanket for use with the second recited known blanket cylinder has a pair of metal-made mounting fixtures on its opposite ends. One of the fixtures is hooked onto one of the jaws bounding the longitudinal edges of the groove in the blanket cylinder. Then, after wrapping the blanket around the cylinder, the other fixture is inserted in the cylinder groove and further in a groove in the tightening spindle. Then the racks are driven by the springs to revolve the spindle in the tightening direction, until the blanket is held fast against the cylinder surface.
It is a definite advantage of this second known blanket cylinder that the blanket thereon is not to develop a slack during printing, because then the springs continue to energize the spindle in the blanket-tightening direction. This advantage is offset, however, by some inherent drawbacks.
First, the second prior art device requires an inconveniently wide groove in the blanket cylinder, wide enough to receive both ends of the blanket together with the mounting fixtures thereon. More precisely, the groove must be wider than the sum of twice the blanket thickness and the total thickness of the fixtures on its opposite ends. Considerable vibration and noise has therefore been generated when, during printing, the meeting ends of the blanket on the blanket cylinder come into direct forced engagement with the meeting ends of the printing plate on the plate cylinder or, via the web, with those of the other blanket on the other blanket cylinder. A reduction of mechanical vibration and noise is of utmost importance as printing presses today are designed to run faster and faster.
Another drawback concerns the complexity of the means for biasing the blanket-tightening spindle in one direction of rotation, such means comprising sets of gear teeth formed on the spindle, racks engaged with the teeth, and compression springs acting on the racks. Additionally, an air conduit together with a source of air under pressure is needed for loosening the blanket by turning the spindle in the opposite direction against the forces of the compression springs.
It is therefore an object of this invention to provide a blanket cylinder so made that the blanket mounted thereto is to stay closely wrapped around the same against the risk of slacking during printing, assuring the production of high quality printings.
Another object of the invention is to attain the first recited object by creating in the blanket cylinder an undercut groove having an entrance slot of materially less width, with a view to the reduction of noise and vibration conventionally generated by reason of an inconveniently wide entrance slot.
Another object of the invention is to make the means for pulling the blanket, as the same is mounted to the blanket cylinder, so as to hold the blanket tightly around the blanket cylinder, far simpler and less expensive than heretofore.
A further object of the invention is to expedite the mounting and dismounting of the blanket to and from the blanket cylinder.
In summary, the present invention concerns an improved blanket cylinder for use in offset lithography, in combination with a blanket assembly which includes a blanket and a carrier sheet therefor. The carrier sheet of the blanket assembly has a major portion to which the blanket is attached, a first end portion bent at an angle to the major portion to provide a first flange, and a second end portion bent twofold to provide a second flange at an angle to the major portion, and a rim at an angle to the second flange.
The blanket cylinder comprises a cylinder having formed therein an undercut groove extending parallel to the axis of the cylinder. The undercut groove has an entrance slot bounded by and between a first longitudinal edge which is cross-sectinally shaped to be positively engaged by the first flange of the carrier sheet in mounting the blanket assembly to the cylinder, and a second longitudinal edge which is cross-sectionally shaped to be temporarily engaged by the rim of the carrier sheet preparatory to insertion of the second flange thereof into the undercut groove. Received in the undercut groove for bidirectional rotation therein, a retainer spindle is cross-sectinally shaped to include a hook for engaging the rim of the carrier sheet in order to pull the second end portion thereof into the undercut groove with the rotation of the retainer spindle in a first direction, and a ledge, circumferentially spaced from and opposed to the hook, for abutment against the rim of the carrier sheet in order to push the second end portion thereof out the undercut groove with the rotation of the retainer spindle in a second direction opposite to the first. Also included is a resilient means for energizing the retainer spindle in the first direction relative to the cylinder in order to hold the blanket assembly fast against the cylinder. Such being the improved construction of the blanket cylinder according to the invention, it is only the opposite end portions of the carrier sheet that are inserted in the undercut groove for mounting the blanket assembly to the blanket cylinder. The entrance slot of the undercut groove can therefore be as narrow as only somewhat more than twice the thickness of the carrier sheet, resulting in appreciable curtailment of the vibration and noise generated upon forced meeting of the slot with the slots of the neighboring cylinders in a web-fed offset printing press.
Upon completion of being mounted to the blanket cylinder, the blanket assembly has its first flange held hooked onto the first longitudinal edge of the entrance slot. The second flange of the blanket assembly is also received wholly in the undercut groove as the rim of the second flange, engaged by the hook of the retainer spindle, is thereby pulled into the groove under the force of the resilient means. The blanket assembly is thus to stay tightly wrapped around the blanket cylinder against the risk of slacking even when placed in forced rolling contact with the plate cylinder and, via the web, with the impression cylinder or another blanket cylinder, during printing.
In one embodiment of the invention, the resilient means biasing the retainer spindle in the blanket-tightening direction takes the form of a pair of torsion springs coiled around the opposite end portions of the retainer spindle. The retainer spindle with such resilient means can be compactly and inexpensively built into the blanket cylinder.
The dismounting of the blanket assembly from the blanket cylinder is just as easy as its mounting. The blanket assembly will come loose on the cylinder simply as the retainer spindle is turned against the bias of the resilient torsion springs, as then the ledge of the spindle will push the rim of the carrier sheet toward the entrance slot of the groove.
In order to expedite such dismounting of the blanket assembly without making the blanket cylinder any more complex in construction, the retainer spindle has a pair of aligned extensions of hexagonal cross section projecting from the opposite ends of the blanket cylinder. Either extension of the spindle may be manually turned by a wrench or spanner, readily available in any printing plant, against the forces of the torsion springs. The above and other objects, features and advantages of the invention and the manner of realizing them will become more apparent, and the invention itself will best be understood, from the following description taken together with the attached drawings showing the preferred embodiments of the invention.